The Hound in question was a witch named Nym Reedham. She had taken Polyjuice Potion in order to take the shape of an anonymous Muggle girl who's hair she'd plucked after standing behind her in a queue.

Nor did she. Nym had simply been told to pass the message on to the insider, who would be wearing a dark red cloak and hat and who would give her a password. The insider had been told to expect a young girl who would be a disguised Hound, so there was no risk of the wrong person being involved.